Forces
Table of Contents
The Nature of Force
Friction and Gravity
Newton’s First and Second Laws
Newton’s Third Law
Rockets and Satellites
Forces
Force
•A force is simply a push or a pull.
•Isaac Newton accurately described the force of all moving
objects in 1687 when he published his thesis.
•We still use these laws as the basis of all physics to this day.
Forces
Units of Force
•When describing the strength of a force we use the SI unit of
Newtons (N)
•The newton is the SI unit for force; it is equal to the amount of
net force required to accelerate a mass of one kilogram at a rate
of one meter per second squared.
•F = ma: or multiplying mass (kg) by acceleration (m/s2)
•Therefore:
1 N = 1 kg·m/s2
Forces
Real World Examples of the Newton
•1 N is the force of Earth's gravity on a mass of about 102 g or
roughly the mass of a small apple
•On Earth's surface, a mass of 1 kg exerts a force of approximately
9.8 N [down] (or 1.0 kilogram-force; 1 kgf=9.80665 N by definition)
•The force of Earth's gravity on a human being with a mass of 70 kg
is approximately 686 N
•It is common to see forces expressed in kilonewtons or kN
(1 kN = 1,000 N)
Forces - The Nature of Force
Combining Forces
The combination of all forces acting on an object is
called the net force.
The net force determines whether an object moves
and also the direction it will move!
Forces - The Nature of Force
Unbalanced Forces
Unbalanced forces acting on an object result in a net force
and cause a change in the object’s motion.
Forces - The Nature of Force
Balanced Forces
Balanced forces acting on an object do not change the
object’s motion.
Forces - The Nature of Force
Asking Questions
Before you read, preview the red headings. In a graphic
organizer like the one below, ask a what or how question for
each heading. As you read, write answers to your questions.
Question
Answer
What is a force?
A force is a push or pull.
What happens when forces
combine?
Forces combine to produce a
net force.
Forces
End of Section:
The Nature of
Force
Forces
Friction
•Friction is the force that is exerted on two surfaces as they
rub against one another
•Friction force acts in the opposite direction of motion!
•The strength of the friction force depends on two things:
1. How hard the surfaces are pushed together
2. What material the surface is made of
Forces
Types of Friction
1. Static friction acts on objects that are at rest
2. Sliding friction occurs when two solid surfaces slide past one another
3. Rolling friction occurs when an object rolls across a surface
4. Fluid friction occurs when a solid object moves through a fluid
Forces
What type of friction is occuring?
Forces
What type of friction is occurring?
Forces
Compare and Contrast
Forces
What type of friction is occurring?
Forces
What type of friction is occurring?
Forces
How were the Egyptian Pyramids Built?
Forces
Gravity
•Gravity is a force that pulls objects toward one another
•Gravity keeps objects in place unless they are acted on by
an outside force!
•Sir Isaac Newton accurately described this in his law of
universal gravitation
•Gravity acts on all objects regardless of their size and or
location in the universe
•This means that all objects are attracted to one another but
there are some rules involved in how much attraction occurs
•This is mostly due to differences in mass
•What is the difference between mass and weight?
•Mass is the amount of matter in an object.
•Weight is the measure of gravitational force on an object.
Forces - Friction and Gravity
Gravity
Two factors affect the gravitational attraction between
objects: mass and distance.
Forces - Friction and Gravity
Gravity
The force of gravity on a person or object at the surface of
a planet is known as weight.
Can you find anything wrong with this picture?
Forces
Gravity and Motion
•An object is said to be in free fall when gravity is the only
force acting on it.
•This results in an unbalanced force and causes the object to
accelerate in a downward motion at 9.8m/s
•This acceleration is not dependent on mass!
•Air resistance is fluid friction that acts in the opposite
direction to motion
•Air resistance is not the same for all objects!
•Terminal velocity occurs when the force of air resistance
equals the weight of the object. Therefore, the object will
continue to fall with a constant velocity with no acceleration
Forces - Friction and Gravity
Free Fall
Use the graph to answer the
following questions.
Forces - Friction and Gravity
Free Fall
Interpreting Graphs:
What variable is on the
horizontal axis? The vertical
axis?
Time is on the horizontal axis,
and speed is on the vertical
axis.
Forces - Friction and Gravity
Free Fall
Calculating:
Calculate the slope of the
graph. What does the slope
tell you about the object’s
motion?
The slope is 9.8. The speed
increases by 9.8 m/s each
second.
Forces - Friction and Gravity
Free Fall
Predicting:
What will the speed of the
object be at 6 seconds?
58.8 m/s
Forces - Friction and Gravity
Free Fall
Drawing Conclusions:
Suppose another object of the
same size but with a greater
mass was dropped instead.
How would the speed values
change?
The speed values would not
change.
Forces - Friction and Gravity
Air Resistance
Falling objects with a greater surface area experience more
air resistance.
Forces
Projectile Motion
•A projectile is any object
projected into space by the
exertion of a force.
•Horizontal velocity does not
affect how fast an object will
fall!
•Therefore, both the red and
yellow ball will hit the ground
at the same time
•On our planet, all objects are
accelerated by the force of
gravity (9.8m/s) towards the
center of the earth.
Forces
What is going on in this diagram?
http://onlinephys.com/kinematics1D.html
Forces - Friction and Gravity
Comparing and Contrasting
As you read, compare and contrast friction and gravity by
completing a table like the one below.
Friction
Gravity
Effect on motion
Opposes motion
Pulls objects toward one
another
Depends on
Types of surfaces
involved, how hard the
surfaces push together
Mass and distance
Measured in
Newtons
Newtons
Forces - Friction and Gravity
Links on Friction
Click the SciLinks button for links on friction.
Forces - Friction and Gravity
Free Fall
Click the Video button to watch a movie about free fall.
Forces
End of Section:
Friction and
Gravity
Forces - Newton’s First and Second Laws
Calculating Force
A speedboat pulls a 55-kg water-skier. The force causes the skier
to accelerate at 2.0 m/s2. Calculate the net force that causes this
acceleration.
Read and Understand
What information have you been given?
Mass of the water-skier (m) = 55 kg
Acceleration of the water-skier (a) = 2.0 m/s2
Forces - Newton’s First and Second Laws
Calculating Force
A speedboat pulls a 55-kg water-skier. The force causes the skier to
accelerate at 2.0 m/s2. Calculate the net force that causes this acceleration.
Plan and Solve
What quantity are you trying to calculate?
The net force (Fnet) = __
What formula contains the given quantities and the unknown
quantity?
a = Fnet/m or Fnet = m X a
Perform the calculation.
Fnet = m X a = 55 kg X 2.0 m/s2
F = 110 kg • m/s2
F = 110 N
Forces - Newton’s First and Second Laws
Calculating Force
A speedboat pulls a 55-kg water-skier. The force causes the skier to
accelerate at 2.0 m/s2. Calculate the net force that causes this
acceleration.
Look Back and Check
Does your answer make sense?
A net force of 110 N is required to accelerate the water-skier. This
may not seem like enough force, but it does not include the force of
the speedboat's pull that overcomes friction.
Forces - Newton’s First and Second Laws
Calculating Force
Practice Problem
What is the net force on a 1,000-kg object accelerating
at 3 m/s2?
3,000 N (1,000 kg X 3 m/s2)
Forces - Newton’s First and Second Laws
Calculating Force
Practice Problem
What net force is needed to accelerate a 25-kg cart
at 14 m/s2?
350 N (25 kg X 14 m/s2)
Forces - Newton’s First and Second Laws
Outlining
As you read, make an outline
about Newton’s first and
second laws. Use the red
headings for the main topics
and the blue headings for the
subtopics.
Newton’s First and Second Laws
I. Newton’s First Law of Motion
A. Inertia
B. Inertia Depends on Mass
II. The Second Law of Motion
A. Changes in Force and Mass
Forces - Newton’s First and Second Laws
More on Newton’s Laws
Click the PHSchool.com button for an activity
about Newton’s laws.
Forces
End of Section:
Newton’s First
and Second
Laws
Forces - Newton’s Third Law
Calculating Momentum
Which has more momentum: a 3.0-kg sledgehammer swung at 1.5
m/s or a 4.0-kg sledgehammer swung at 0.9 m/s?
Read and Understand
What information have you been given?
Mass of smaller sledgehammer = 3.0 kg
Velocity of smaller sledgehammer = 1.5 m/s
Mass of larger sledgehammer = 4.0 kg
Velocity of larger sledgehammer = 0.9 m/s
Forces - Newton’s Third Law
Calculating Momentum
Which has more momentum: a 3.0-kg sledgehammer swung at 1.5
m/s or a 4.0-kg sledgehammer swung at 0.9 m/s?
Plan and Solve
What quantities are you trying to calculate?
The momentum of each sledgehammer = __
What formula contains the given quantities and the unknown
quantity?
Momentum = Mass X Velocity
Perform the calculation.
Smaller sledgehammer = 3.0 km X 1.5 m/s = 4.5 kg•m/s
Smaller sledgehammer = 4.0 km X 0.9 m/s = 3.6 kg•m/s
Forces - Newton’s Third Law
Calculating Momentum
Which has more momentum: a 3.0-kg sledgehammer swung at 1.5
m/s or a 4.0-kg sledgehammer swung at 0.9 m/s?
Look Back and Check
Does your answer make sense?
The 3.0-kg hammer has more momentum than the 4.0-kg one. This
answer makes sense because the 3.0-kg hammer is swung at a
greater velocity.
Forces - Newton’s Third Law
Calculating Momentum
Practice Problem
A golf ball travels at 16 m/s, while a baseball moves at 7
m/s. The mass of the golf ball is 0.045 kg and the mass
of the baseball is 0.14 kg. Which has the greater
momentum?
Golf ball: 0.045 kg X 16 m/s = 0.72 kg•m/s
Baseball: 0.14 kg X 7 m/s = 0.98 kg•m/s
The baseball has greater momentum.
Forces - Newton’s Third Law
Calculating Momentum
Practice Problem
What is the momentum of a bird with a mass of 0.018 kg
flying at 15 m/s?
0.27 kg•m/s (0.018 kg X 15 m/s = 0.27 kg•m/s)
Forces - Newton’s Third Law
Conservation of Momentum
In the absence of friction, momentum is conserved when two
train cars collide.
Forces - Newton’s Third Law
Momentum Activity
Click the Active Art button to open a browser window and
access Active Art about momentum.
Forces - Newton’s Third Law
Previewing Visuals
Before you read, preview Figure 18. Then write two
questions that you have about the diagram in a graphic
organizer like the one below. As you read, answer your
questions.
Conservation of Momentum
Q. What happens when two moving objects collide?
A. In the absence of friction, the total momentum is the same before and
after the collision.
Q. What is the momentum of an object?
A. Its mass multiplied by its velocity
Forces
End of Section:
Newton’s Third
Law
Forces - Rockets and Satellites
What Is a Satellite?
A projectile follows a curved path because the horizontal and
vertical motions combine.
Forces - Rockets and Satellites
What Is a Satellite?
The faster a projectile is thrown, the father it travels before it
hits the ground. A projectile with enough velocity moves in a
circular orbit.
Forces - Rockets and Satellites
What Is a Satellite?
Depending on their uses, artificial satellites orbit at different
heights.
Forces - Rockets and Satellites
Identifying Main Ideas
As you read the section “What Is a Satellite?” write the main
idea in a graphic organizer like the one below. Then write
three supporting details that further explain the main idea.
Main Idea
A satellite stays in orbit due to…
Detail
its inertia
Detail
Earth’s gravity
Detail
Earth’s shape
Forces
End of Section:
Rockets and
Satellites
Forces
Graphic Organizer
Type of Friction Occurs When
Example
Static
An object is not
moving
Friction between an
unmoving book and
desk
Sliding
Two solid surfaces
slide over each
other
Rubber pads on a
bicycle’s brakes
Rolling
An object rolls
across a surface
Ball bearings in
skateboard wheels
Fluid
A solid object moves
Air resistance
through a fluid
Forces
End of Section:
Graphic Organizer